Hypertensive kidney disease commonly progresses. The primary objective of the AASK (African American Study of Kidney Disease and Hypertension) Cohort Study is to determine prospectively the course of kidney function and risk factors for kidney disease progression in African-Americans with hypertensive kidney disease who receive recommended antihypertensive therapy. A secondary objective is to determine the occurrence of cardiovascular disease and assess its risk factors. The AASK Cohort Study is a prospective, observational study that is an extension of the AASK trial. The AASK trial tested the effects on kidney function of 3 medications used as initial antihypertensive therapy (ramipril, metoprolol and amlodipine) and 2 levels of blood pressure control. Of the 1,094 trial participants, approximately 650 to 700 individuals who have not reached end stage renal disease (ESRD) will likely enroll in the Cohort Study. Risk factors to be studied include environmental, genetic, physiologic, and socioeconomic variables. The primary renal outcome is a composite clinical outcome defined by doubling of serum creatinine, ESRD, or death. Medication treatment for hypertension, beginning with the angiotensin converting enzyme inhibitor ramipril, is offered to all participants. In this fashion, the study directly controls two of the major determinants of kidney disease progression (treatment of hypertension and use of renoprotective, antihypertensive medication). The minimum duration of follow-up in the Cohort Study is 5 years (total of 9 to 12 years, including the period of the AASK trial). Ultimately, data from the AASK Cohort Study should enhance our understanding of the risk factors and processes that determine the progression of kidney disease. Such results might eventually lead to new strategies that delay or prevent ESRD.